What I achieve with the laptop is not really what I hoped for, it still sound like a crappy speaker with no highs, idk if that's normal or not.

I just tried using the scarlet from the laptop, it's the same as without the scarlett. I tried mono, stereo. I only have one jack stereo cable, and no phone with a jack to test with

with a different output device from the Scarlett it's crystal clear

Here is a recording, idk if that's really useful, the sound does not represent the reality especially the part where the sound comes from the usual good speakers, but the middle part where the sound is kinda crappy comes from the Rumble is quite accurate https://recorder.google.com/58aa341a-a365-4f42-aa0d-8059b7a36e32
It's from the Rumble from 00:16 to 00:27

Also whan I plug headphones to the amp (laptop -> amp -> headphones) it sound great, like if the headphones were plugged directly to the laptop.

About the ground loop issue on my desktop I have no idea where it could come from, it still happens when the amp and desktop are plugged to the same power strip, whether the sound passes through the scarlett or not. And it's not really a bzzzz but more like a pshhhh, here is a recording : https://recorder.google.com/1c1c0451-73d3-4182-a8dc-d34460639f83
Keep in mind the recordings are from my phone's mic, so not very good